Field of Invention
This invention relates to near field RF communicators and more particularly to methods and apparatus for detecting peaks in a received RF signal in such communicators.
Related Art
It is desirable to derive the envelope of a received RF signal in a near field RF communicator for use in demodulation of the signal. To achieve this it has been proposed to employ diode coupled transistors to provide an envelope signal from an AC voltage. However the voltage drop across transistors coupled in this way (“diode drop”) means that this solution is not optimal. It has also been proposed to differentiate an AC voltage to provide an indication of the voltage extremes (minima and maxima) of the AC voltage. However this method is known to suffer from instability, particularly where noise is present on a signal. It has also been proposed to provide DSP circuits to estimate the envelope. However, typical DSP circuits require supply voltages in excess of the AC voltage to be sensed and therefore are inappropriate for use in field powered near field communicators.
The inventors in the present case have appreciated that by using zero crossing detection improved peak detection can be provided if an accurately known delay of one quarter of a cycle of the RF carrier frequency is available to the demodulator.
Prior art methods of determining clock cycle period and providing a delay based on that period are unreliable or require the application of large voltages to sweep the control voltage applied to a delay locked loop. Where power is to be derived from a received RF signal such methods of providing a delay are not appropriate.